Prussian Blue was the first of the modern pigments to be discovered. The brightness and intensity of color were very sought after. The inexpensive cost was another huge selling point. It was used extensively in the 18th and 19th centuries. In my c.1800 farmhouse in upstate NY, every single inch of trim, in every room, throughout all 3 floors was originally painted with very intense Prussian blue milk paint. This was a trend that would fade by the 1850s when other chemically derived pigments were invented, and other colors became cheaper to produce. Though, you mention blue radiators, which wouldn't be original to the 1840s, so maybe someone who lived there at some point really liked blue.